By definition, composite materials include two or more phases having different physical characteristics. Many composites incorporate fibers, typically of a relatively rigid material, in a matrix of another material which ordinarily is less rigid. For example, polymers are often reinforced with fibers of glass, ceramic or carbon, whereas metals may be reinforced with ceramic fibers.
One polymer used in commercial composite structures is isotactic polypropylene. As is known, isotactic polypropylene can take on a number of crystal forms. The most common of these crystal forms is the monoclinic or "alpha" form, and this crystal form is predominant in most commercially processed isotactic polypropylene composites. A second form of isotactic polypropylene that has been widely documented in the literature is the hexagonal or "beta" form. This form, which can be induced through the incorporation of red quinacridone pigments to a polypropylene melt, has a lower stiffness and higher toughness that the alpha form. Purportedly, the beta form of polypropylene is useful in forming films and thermoforming articles from such films.
Composites, of course, present unique problems not encountered with uniform, single phase materials and the design of a composite material often involves balancing competing considerations. For example, there is an inverse relationship between stiffness and toughness in a polymer/fiber composite. The strength and stiffness in composites containing a given fiber reinforcement is directly related to the strength of the bond between the fiber and the polymer matrix. On the other hand, toughness is inversely related to the strength of the fiber-matrix bond. This tradeoff between strength and toughness is a major concern to those who develop and design with these materials.
Attempts have been made to enhance the performance of composites by providing an "interphase" between the fibers and the surrounding matrix material. As distinguished from an interface of molecular scale dimensions, an interphase constitutes a distinct phase having physical properties different from those of the fiber and different from those of the matrix. One method for making such a composite, as disclosed in U.S. Pat. No. 5,288,555, is to coat the fiber material with a nucleating agent and then process the coated fiber with the base polymer. A suggested polymer is polypropylene and a suggested nucleating agent is a quinacridone pigment, in which case the net result after processing is a composite in which the material is alpha polypropylene and the fibers in the matrix are coated with an interphase of beta polypropylene.